1. Field of the Invention
The present invention relates to a polycrystalline infrared transmitting barium fluoride sintered body excellent in transmission to infrared rays, which is suitable for use as an infrared ray transmitting (optical) material or as infrared optical components etc. used in infrared equipment.
2. Description of the Prior Art
In recent years, various types of infrared equipment which detect infrared rays from heat radiated or scattered from a body have developed. For example, infrared sensors for accurately checking the position of an object as anticrime detectors, night vision devices capable of observing an object in the dark and thermometers for measuring the temperature of an object and its temperature distribution and the like instruments have been developed and they are beginning to enjoy great popularity.
Infrared optical components, such as window materials, lenses, prisms etc. which are used in these infrared equipment are required to be made of materials which will transmit infrared rays in the required wave length band. Materials hitherto generally used as such infrared ray transmitting materials are such single crystalline materials as germanium (Ge), silicon (Si), potassium chloride (KCl), calcium fluoride (CaF.sub.2) and barium fluoride (BaF.sub.2). These single crystalline materials were high-priced, because a long time was required for their manufacture and large-sized products were manufactured only with difficulty. Because of their liability to cleavage, they could hardly be said to have adequate mechanical strengths.
On the other hand, recently the development of infrared transmitting materials by use of the CVD (chemical vapor deposition) process has been advanced, with a result that polycrystalline materials e.g. zinc selenide, zinc sulfide etc. have been made available. Because the rate of growth of these polycrystalline materials by the CVD process is slow, their price rise is unavoidable and most of their uses have been as high priced parts, for example, as optical components of carbon dioxide lasers.
These polycrystalline materials, having high refractive indices and manifesting large loss by surface reflection, give not very high in-line transmissions, approx. 70% for ZnSe in the form of its 3 mm thick test piece and approx. 73% for ZnS, for example, being the maxima. Accordingly, it is a general practice to apply some antireflection coating on them, when they are used as optical components like optical windows etc., only to serve as a factor of cost increase.
Further, as described in U.S. Pat. No. 3,431,326, methods for producing polycrystalline infrared transmitting materials such as magnesium fluoride (MgF.sub.2) or barium fluoride (BaF.sub.2) or the like by the simple hot press process have been proposed. With such polycrystalline materials obtained by the process above mentioned, large absorption peaks are recognized in the practically useful region of 8-11 .mu.m wave lengths, as shown in FIG. 4. and FIG. 5. Such absorption peaks are deleterious, for they lower the sensitivity of infrared equipment.